1. Field of the Invention
This invention relates to an adhesive for a printed circuit board and to a process for producing a printed circuit board using the adhesive.
2. Related Art Statement
It is known that in a process for producing a printed circuit board comprising forming an adhesive layer on the surface of a laminate consisting of insulating substrates and then forming a circuit and through-holes there by electroless plating, the adhesive layer is roughened with an oxidizing agent in order to enhance the adhesion between the plated circuit and the adhesive layer. As disclosed in Japanese Patent Application Kokoku No. 63-10,752; Japanese Patent Application Kokai No. 63-297,571; Japanese Patent Application Kokai No. 64-47,095; Japanese Patent Application Kokai No. 3-18,096 and the like, almost all adhesives contain a rubber component such as acrylonitrile-butadiene rubber or the like, and an aqueous chromic acid-sulfuric acid solution is used as the oxidizing agent, and the roughening of the surface of the adhesive layer is effected by dissolving the rubber component.
However, the adhesive containing a rubber component is low in reliability of heat resistance, electrical insulation properties, metal-migration resistance, etc., and there is a problem when it is applied to a printed circuit board requiring a high reliability and a printed circuit board having a high density plated circuit.
There are also some other methods of roughening the adhesive layer such as a method in which a dispersion of an inorganic fine powder such as silica, calcium carbonate or the like in a resin matrix having excellent heat resistance such as an epoxy resin, a phenol resin, a melamine resin or the like is used as an adhesive and the inorganic fine powder in the adhesive is selectively dissolved in a specific chemical; a method as described in Japanese Patent Application Kokai No. 1-29,479 which comprises dispersing, in an epoxy resin matrix, a cured epoxy resin fine powder having a different solubility in an oxidizing agent from the solubility of the epoxy resin matrix in the oxidizing agent and selectively dissolving the cured epoxy resin fine powder in the oxidizing agent; and the like.
However, in the above-mentioned methods, it is necessary to uniformly disperse, in an adhesive varnish, a fine powder (having a particle size of an order of several microns) in such an amount as to obtain a satisfactory roughening effect in order to obtain a good dimension precision, and this is technically difficult.
In the above methods, a strong oxidizing agent such as aqueous chromic acid-sulfuric acid solution is used in cases other than the case in which the fine inorganic powder is selectively dissolved off in the specific chemical, so that there are severe problems in respect of working atmosphere, safety and health. Moreover, there are such problems that when the adhesive layer formed on a substrate through which holes for forming plated through-holes have been bored is toughened, excessive roughened surface is formed on the inner walls of the holes, and hence, the connection reliability of the plated through-holes is lowered; and the electroless plating solution is apt to permeate the substrate along the glass cloth, and the insulation reliability between the plated through-holes is also greatly lowered.
As a method of solving the above problems, as stated in Japanese Patent Application Kokoku No. 5-4,840, it has been proposed to cover the surface of a substrate having a toughened adhesive layer with a protective mask and then bore holes for forming plated through-holes.
However, the steps of covering with the protective mask and removing the protective mask are increased, and furthermore close attention must be paid so as to keep the roughened shapes formed in the adhesive layer in the state just after the roughening treatment.
Thus, in the conventional adhesives for printed circuit board and the conventional processes for producing a printed circuit board using the said adhesives, there are such problems that in the case of an adhesive containing a rubber component, the reliability of the resulting printed circuit board is lowered, that in the case of an adhesive comprising a resin matrix in which a fine powder to be selectively dissolved in an oxidizing agent is dispersed, it is very difficult to produce the fine powder with good precision and disperse the same uniformly, that since an aqueous chromic acid-sulfuric acid solution is used as the oxidizing agent for roughening the adhesive layer, working atmosphere, safety and health are greatly deteriorated, and that when a multilayer substrate through which holes for forming plated through-holes have been bored is roughened, the inner walls of the holes are also roughened, and hence the reliability of the plated through-holes is lowered.